Republican presidential candidate and former House Speaker Newt Gingrich used campaign funds for personal gain, a left-leaning watchdog group alleged in a Federal Election Commission complaint filed Monday.

Citizens for Responsibility and Ethics in Washington (CREW) has called for an FEC investigation to determine whether one of Gingrich’s companies, Gingrich Productions, Inc., made tens of thousands of dollars in illegal contributions to his presidential campaign.

A Dec. 6 Washington Post report detailed how Gingrich’s campaign paid him $42,000 for the use of his extensive mailing list. The expense was not listed on the campaign’s third-quarter FEC disclosures.

The Gingrich campaign said the failure to disclose the payment was a mistake, but maintains it stayed well within the law.

“If the FEC considers the complaint, they will find that the rules are being followed and published regulations are being enforced,” Gingrich spokesperson R.C. Hammond said in a statement Monday.

Federal election law forbids candidates from using campaign resources to make a profit. It also prohibits candidates from using corporate funds to subsidize their campaigns.

CREW alleges Gingrich converted campaign funds for his personal use by accepting the $42,000 payment to use the mailing list. Although the campaign argues the list belonged to Gingrich personally, the former Speaker did not list it among his assets in his latest personal finance disclosure. Crew argues the list then belongs to Gingrich Productions.

CREW also asked the FEC to investigate whether Gingrich Productions illegally subsidized Gingrich’s campaign by holding campaign events in conjunction with his book signings.

“Newt Gingrich will do anything to make a buck, even sell his own mailing list to his campaign,’ CREW Executive Director Melanie Sloan said. “He has a long history of playing fast and loose with ethics rules so it should surprise no one to learn he is at it again.”

Gingrich was hit with a flurry of ethics charges during his time as Speaker of the House. Eighty-four ethics charges were filed against him. All were dropped, but the House still levied a $300,000 fine against Gingrich for ethical lapses related to a politics-heavy college course administered by a non-profit organization. It was the first time in House history that a sitting speaker was sanctioned.